Our long-term goal is to understand how stable differences in developmental ability are established in the neural crest (NC) cell lineage. When trunk NC cells segregate from neural tube epithelium in vivo, they enter a cell-free, extracellular matrix-rich "migration staging area" (MSA), and then disperse on two distinct migration pathways. Trunk NC cells that disperse early on a medioventral ("medial") pathway between the neural tube and the somites give rise to both neuronal and non- neuronal derivatives. These include sensory, autonomic and enteric neurons, Schwann sheath cells and glia of the peripheral ganglia, and perhaps some late-developing melanocytes in distal locations. In contrast, trunk NC cells that disperse later on a dorsolateral ("lateral") pathway never give rise to neuronal derivatives, but differentiate only as melanocytes (and perhaps other non-neuronal) crest derivatives. We have found that crest-derived melanocyte precursors express c-kit, the receptor for Steel Factor (SLF), and transiently depend on SLF function for survival in vitro. Previous work, by us and others, has also shown that differentiation of melanocytes and other non-neuronal, crest-derived cells is affected both by SLF and by platelet-derived growth factor (PDGF) function, mediated by the receptor PDGFRalpha. To explain why melanocytes arise primarily (or solely) from NC cells on the lateral pathway of mouse embryos, we hypothesize that developmentally-distinct crest-derived cells, which appear at late stages of crest cell dispersal from the MSA , selectively follow the lateral crest-migration pathway where they encounter localized growth/survival cues that they require. To test this hypothesis, we will examine murine embryos during crest cell dispersal, and cultured crest cells at corresponding developmental stages, to learn (a) when and where crest-derived melanocyte precursors first acquire the receptors for Steel factor (c-kit) and platelet-derived growth factor, A- chain (PDGFRalpha), (b) when and where crest cells encounter SLF and PDGF during their initial dispersal in embryonic interstitial spaces, and (c) how mutational perturbation of SLF presentation affects developmental behavior of melanocyte precursors. Specifically, we will: 1. characterize the expression of c-kit and PDGFRalpha in late-migrating (non-neurogenic) crest-derived cells in vitro; 2. determine the distribution of SLF and PDGF on the medial and lateral crest migratory pathways in the embryonic trunk, and the pattern of dispersal of crest-derived melanocyte precursors on these two pathways; and concurrently, 3. characterize the pattern of dispersal and the fate of crest-derived melanocyte precursors when SLF and PDGF function are mutationally perturbed in vivo.